The present invention relates to a magnetic recording medium or, more particularly, to a magnetic recording medium in which the magnetic coating layer on a substrate is formed by using a specific resinous binder capable of exhibiting excellent binding power for the ferromagnetic particles dispersed as well as a magnetic coating composition therefor.
As is known, magnetic recording media such as magnetic recording tapes, floppy discs and the like are prepared usually by coating the surface of a substrate made of a plastic such as polyesters with a magnetic coating composition which is a dispersion of ferromagnetic particles in a synthetic resin as a binder or vehicle.
Examples of the ferromagnetic particles used in magnetic recording media include particles of iron oxide, e.g., .gamma.-Fe.sub.2 O.sub.3 and Fe.sub.3 O.sub.4, optionally, doped with or adsorbing cobalt ions, chromium oxide CrO.sub.2, and needle-like fine metallic particles of iron, cobalt, an alloy of iron and cobalt optionally containing nickel. Along with prevalence of household video tape recorders and the trend of cassetted audio tapes toward higher and higher performance in recent years, magnetic recording media are required to be compatible with the demand for an increased density of signal recording and higher playback output in short-wavelength recording. It is important in order to satisfy these requirements that the ferromagnetic particles have a finer particle size distribution and hence a greatly increased magnetic moment. This situation results in an increased difficulty in obtaining uniform distribution of the ferromagnetic particles in the resinous binder because the particles readily agglomerate as a consequence of the increased magnetic moment.
In view of the above described technical problems which could be solved by increasing the affinity between the ferromagnetic particles and the resinous binder, extensive investigations are now under way to impart the resin binder with hydrophilicity which is possessed by the ferromagnetic particles of a metallic oxide, metal or alloy.
For example, a proposal has been made to introduce hydrophilic functional groups such as carboxyl and hydroxy groups into a copolymer of vinyl chloride and vinyl acetate currently in practical use as a binder resin. Several scientific articles of "Magnetic tapes and high polymers" appearing in "Nippon Secchaku Kyokaishi" (Journal of Japanese Association of Adhesion), volume 17, No. 4 (1981), pages 155 to 162, are directed to the technical problem of introducing various kinds of hydrophilic groups into polymers as a binder in magnetic coating compositions used for magnetic recording media. According to one of the articles, several kinds of hydrophilic groups are rated in a decreasing order as shown below relative to the types of anchor segments and the effectiveness thereof: EQU --SO.sub.3 H&gt;--COOH&gt;--OH&gt;--N.dbd.&gt;(epoxy)&gt;--CN.
Effectiveness of the sulfonic acid groups --SO.sub.3 H introduced into a binder resin is also known from Japanese Patent Kokai No. 58-108032, according to which examples of suitable resins as a binder of ferromagnetic particles include copolymers obtained by the copolymerization of a monomer such as a vinyl carboxylate, vinyl chloride, vinylidene chloride, acrylonitrile, styrene and the like and a polymerizable unsaturated sulfonic acid such as vinyl sulfonic acid, vinylbenzene sulfonic acid, 2-acrylamide-2-methyl propane sulfonic acid and the like.
Although a remarkable improvement can indeed be obtained in the dispersibility of ferromagnetic particles by the introduction of sulfonic acid groups, for example, into a copolymer of vinyl chloride and a vinyl carboxylate, this means is not always quite satisfactory in respect of the relatively poor surface gloss of the magnetic recording media prepared from a modern high-performance ferromagnetic powder with such a resin as a binder, as well as a noticeable decrease of the saturation magnetization of the magnetic recording media with the lapse of time. The effectiveness of introduction of carboxyl and hydroxy groups into a binder resin is insufficient in respect of the desired improvement of dispersibility of the ferromagnetic particles.
Japanese Patent Kokai No. 57-44227 proposes a binder resin which is a copolymer composed of vinyl chloride moiety, vinyl acetate moiety and vinyl alcohol moiety with the hydroxy group of the vinyl alcohol moiety modified with a hydrophilic group such as --SO.sub.3 M, --OSO.sub.3 M and the like, M being an atom of metal. An improvement is indeed obtained with the binder resin of this type relative to the dispersibility of the ferromagnetic particles and dusting but the resin is liable to cause dehydrochlorination reaction so that it is disadvantageous in respect of the stability or long-term durability. In particular, the resin is responsible for the degradation of the magnetic properties such as the density of residual magnetic flux with the lapse of time when the ferromagnetic particulate material is formed of an alloy such as those of iron and cobalt. The reason for this very serious defect of the resin is presumably that degradation of the polymer takes place in the course of the manufacturing process of the resin which is performed under severe reaction conditions such that the hydroxy groups in the starting polymer, which is a vinyl chloride-based polymer having hydroxy groups introduced thereinto, are modified by the reaction with a chlorine-containing sulfonic acid in the form of a metal salt followed by the introduction of a sulfonic acid group in the form of a metal salt.
In view of the above described problems, the inventors have previously proposed an improved magnetic recording medium in Japanese Patent Kokai No. 61-39927, which is prepared by providing a substrate with a magnetic coating layer of a coating composition containing ferromagnetic particles dispersed in a copolymeric binder resin composed of (1) vinyl chloride moiety, (2) vinyl alcohol moiety, (3) vinyl carboxylate moiety having 8 to 16 carbon atoms in a unit and (4) moiety of polymerizable unsaturated sulfonic acid optionally or at least partly in the form of a metal salt. The resin of this type is indeed very superior to the conventional ones but still has a problem, especially, when the ferromagnetic particles dispersed therein have a very fine particle size distribution, for example, with a specific surface area of 35 m.sup.2 /g or larger as determined by the BET method, although the magnetic recording medium prepared therefrom has good surface properties. Namely, the magnetic coating composition prepared by compounding the resin and such a fine ferromagnetic powder may have an unduly high consistency as a consequence of the poor dispersibility and impregnation performance of the particles in the resinous vehicle so that difficulties are encountered during coating of the substrate surfaces using the coating composition to form a fully uniform coating layer at a high velocity. Although a possibility of high-velocity coating can be obtained by using a resin having a decreased degree of polymerization to give a coating composition of lower consistency, the magnetic coating layer formed by using such a low-molecular resin as the binder is somewhat fragile so that the durability of the magnetic recording medium is unavoidably decreased.